1. Field of the Invention
This invention relates generally to solid oxide fuel cells (SOFCs) which contain an electrolyte between two electrodes and a new and improved interlayer material used between one of the electrodes and the electrolyte.
2. Background Information
High temperature, solid electrolyte, electrochemical generators employing interconnected electrochemical fuel cells convert chemical energy into direct current electrical energy at temperatures of about 800xc2x0 C. to 1200xc2x0 C. Such solid electrolyte fuel cells and multi-cell generators are well known; their fuel electrodes, air electrodes, solid oxide electrolytes and interconnection configurations are taught in U.S. Pat. No. 4,490,444 (Isenberg). Each electrochemical fuel cell typically includes a self-supporting porous air electrode or cathode made of, for example, doped lanthanum manganite (LaMnO3). A dense, gas-tight, solid electrolyte is deposited on, and surrounds, the outer periphery of the air electrode. The electrolyte is made of, for example, yttria stabilized zirconia (YSZ)xe2x80x94of the formula(ZrO2)0.9(Y2O3)0.1xe2x80x94about 0.001 mm to 0.1 mm thick. A porous fuel electrode or anode is deposited on, and substantially surrounds, the outer periphery of the solid electrolyte and is made of nickel-zirconia cermet about 0.1 mm thick. Both the solid electrolyte and the fuel electrode are discontinuous to allow for inclusion of an interconnect on the air electrode, so as to provide means to electrically connect adjacent electrochemical fuel cells. A dense, gas-tight interconnect is deposited on a selected radial segment of the air electrode, at the portion that is discontinuous in the electrolyte and fuel electrode, and is made of calcium, strontium, or magnesium doped lanthanum chromite (LaCrO3).
In the past, protective interlayer materials have been suggested for a variety of reasons. In U.S. Pat. No. 5,516,597 (Singh et al.), a niobium and or tantalum doped CeO2 was suggested between the interconnect and the air electrode as a protective layer, to prevent interconnect degradation during long term electrochemical operations. Singh et al., in U.S. Pat. No. 5,106,706, recognized problems of oxygen loss from air electrode particles in contact with the solid electrolyte and the need to increase the active area for electron exchange reactions with oxygen at the electrode-electrolyte interface, suggesting a porous discontinuous interlayer of at least one of single-phase cerium oxide or praseodymium oxide. These oxides were in the form of particles having diameters from 0.01 micrometer to 0.1 micrometer and were disposed between the electrolyte and air electrode to prevent intrusion of electrolyte into the porous air electrode surface and provide abundant nucleation sites for electrolyte formation.
Li Baozhen et al., in U.S. Pat. No. 5,993,989, also recognized problems of interaction between the lanthanum oxide-based air electrode and the zirconia-based electrolyte to form poorly conducting compounds increasing cell resistance and air electrode polarization. There, use of ceria as an interface material was felt to provide substantial thermal expansion mismatch, and a terbia/yttria stabilized zirconia was suggested as an alternate interface material. A Sc2O3-stabilized zirconia electrolyte was also suggested. In most cases in these previous patents the preferred air electrode composition was a Ca and Ce doped LaMnO3 as taught by U.S. Pat. Nos. 4,562,124 and 5,686,198 (Ruka and Kuo et al., respectively).
Virkar et al., in U.S. Pat. No. 5,543,239, concerned with the need for an enhanced charge transfer at the three-phase boundary of electrode/electrolyte/gas phase in electrochemical devices, suggested an electrode configuration having a solid electrolyte center, such as YSZ, covered on both sides with a porous surface layer of dense electrolyte material such as either YSZ or Bi2O3 interdispersed with an electrocatalyst. The suggested electrocatalysts included: silver; platinum; palladium; rhodium; iridium; ruthenium; (La1xe2x88x92xSrx) MnO3 where x is 0-0.5; (La1xe2x88x92xSrx) CoO3, wherein x is 0 to 0.6; (La1xe2x88x92xSrx) (Co1xe2x88x92yFey)O3, wherein x is 0 to 0.4 and y is 0 to 0.8; In2O3xe2x80x94PrO1.83xe2x80x94ZrO2, having composition ratios of In2O3 of 0-90%, PrO1.83 of 10-60% and ZrO2 of 0 to 50%; TbO2 being 35 to 40% doped YSZ; SnO2 being 0 to 20% doped with In2O3; ZrO2 being 0 to 40% doped with In2O3; Sm0.5 Sr0.5 CoO3; La0.6Ca0.4MnO3; Y1xe2x88x92xCaxFeO3, wherein x is 0 to 1; SrCo1xe2x88x92xFeO3, wherein x is 0.2 to 0.8; and TiO2 being 0 to 30% doped with YSZ.
In U.S. Pat. No. 5,629,103 (Wersing et al.), electrochemically active interface layers were applied between solid oxide electrolyte and both the cermet anode and the air electrode cathode in a fuel cell, to improve bonding and interface surface area. The interlayer next to the cermet anode was a doped zirconium or cerium oxide, with a 1 micrometer to 3 micrometer thick interlayer of ionically and electronically conducting air electrode cathode material on the cathode side, such as one of (La.Sr.Ca)(Mn.Co.Ni)O3.
In addition to inefficiencies at the electrolyte-air electrode interface in fuel cells with regard to electrochemical activity, three-phase boundary charge transfer, oxygen loss and deleterious reactions between the two components, new problems relating to output per cell have arisen when the fuel cell is run at low temperatures, in order to try to develop even more environmentally clean SOFC power generation. Current use of a single phase cerium oxide interlayer, at thicknesses up to 2 micrometers, (as taught in U.S. Pat. No. 5,106,706, discussed previously) does not appear to provide an optimized electrochemical reaction area. What is needed is a means to decrease the charge transfer polarization further and to improve performance over what is possible through use of a single phase cerium oxide interlayer.
Therefore, it is a main object of this invention to provide fuel cells having improved performance at lower operating temperatures. It is another object of this invention to provide fuel cells having a higher operating efficiency, reduced fuel consumption per unit of power generated, and reduced cost.
These and other objects of the invention are accomplished by providing a tubular, solid electrolyte electrochemical fuel cell which can operate at temperatures of about 800xc2x0 C. to 1000xc2x0 C. which comprises: a first tubular air electrode consisting essentially of a cerium and calcium doped LaMnO3 material; a solid, tubular electrolyte consisting essentially of stabilized zirconia, disposed on a first portion of the air electrode; a tubular fuel electrode consisting essentially of a cermet material disposed on a portion of the solid electrolyte; and a tubular sintered discrete interlayer disposed between and contacting the electrolyte and air electrode, the interlayer consisting essentially of at least a two-phase mixture of 20 wt % to 80 wt % of a material selected from the group consisting of scandia stabilized zirconia particles, yttria stabilized zirconia particles, doped cerium oxide particles, and mixtures thereof, having a particle size in the range of 0.5 micrometer to 5 micrometers, and 20 wt % to 80 wt % of a material selected from the group consisting of doped lanthanum manganite particles, doped lanthanum chromite particles and platinum particles, having a particle size in the range of 1 micrometer to 5 micrometers, where at least 50% of the particles in the interlayer are less than 3 micrometers, the sintered interlayer having a thickness of 10 micrometers to 40 micrometers and a porosity between about 5% to 50%.
Preferably, the electrolyte is selected from yttria stabilized zirconia or scandia stabilized zirconia, deposited by either plasma arc spraying, or flame spraying. Voltage versus current density characteristics at the air electrode-electrolyte interface, at 800xc2x0, 900xc2x0 and 1000xc2x0 C., show a 5% to 10% improvement using the interlayer of this invention over state-of-the art single phase cerium oxide interfaces. The term xe2x80x9ctubularxe2x80x9d is herein defined as including common circular fuel cells with circular air electrodes, as well as the flattened circular design having internal ribs shown in FIG. 3, as well as other flattened and elongated oval designs discussed later. The first portion of the air electrode which the electrolyte is deposited, is that portion not to be covered by the interconnect material.